Power stealing for a wireless-enabled thermostat

ABSTRACT

Disclosed are exemplary embodiments of apparatus, systems and methods for power stealing for a wireless-enabled thermostat. In an exemplary embodiment, a wireless-enabled thermostat generally includes a control having a wireless network interface that intermittently connects the thermostat in a wireless network in accordance with a duty cycle, the duty cycle having a connect time in which the thermostat is connected in the wireless network and a sleep time in which the thermostat is not connected in the wireless network. A power stealing circuit of the thermostat steals power through an “on-mode” load of a climate control system to charge a capacitor or other energy storage device to provide the power for the wireless network interface. The control adjusts at least the sleep time in accordance with a time for charging the capacitor or other energy storage device to a threshold voltage.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 13/886,797 filed May 3, 2013 which claims the benefit of U.S.Provisional Application No. 61/814,776 filed on Apr. 22, 2013. Theentire disclosures of the above applications are incorporated herein byreference.

FIELD

The present disclosure generally relates to power stealing, and moreparticularly (but not exclusively) to power stealing for awireless-enabled thermostat.

BACKGROUND

This section provides background information related to the presentdisclosure which is not necessarily prior art.

Digital thermostats in climate control systems typically havemicrocomputers and other components that continuously use electricalpower. A number of currently available thermostats also have wirelesscommunication capabilities. Such a thermostat (referred to herein as a“wireless-enabled” thermostat) may be wirelessly connected, e.g., withone or more sensors in a network to provide climate control in a home orother structure.

SUMMARY

This section provides a general summary of the disclosure, and is not acomprehensive disclosure of its full scope or all of its features.

According to various aspects, exemplary embodiments are disclosed ofapparatus, systems and methods for power stealing for a wireless-enabledthermostat. In an exemplary embodiment, a wireless-enabled thermostatfor use in a climate control system generally includes a control havinga wireless network interface configured to intermittently connect thethermostat in a wireless network in accordance with a duty cycle, theduty cycle having a connect time in which the thermostat is connected inthe wireless network and a sleep time in which the thermostat is notconnected in the wireless network. A power stealing circuit of thethermostat is configured to steal power through an “on-mode” load of theclimate control system and to charge a capacitor or other energy storagedevice to provide the power for the wireless network interface. Thecontrol is configured to adjust at least the sleep time in accordancewith a time for charging the capacitor or other energy storage device toa threshold voltage.

In another example embodiment, a wireless-enabled thermostat for use ina climate control system includes a control having a wireless networkinterface configured to intermittently connect the thermostat in awireless network in accordance with a duty cycle, the duty cycle havinga connect time in which the thermostat is connected in the wirelessnetwork and a sleep time in which the thermostat is not connected in thewireless network. A power stealing circuit is configured to steal powerthrough an “on-mode” load of the climate control system and to charge anenergy storage device over a time period varying with current throughthe “on-mode” load, to provide the power for the wireless networkinterface. The control is configured to adjust at least the sleep timein accordance with the varying time period.

Also disclosed, in one embodiment, is an example method that generallyincludes controlling a wireless network interface of the thermostat tointermittently connect the thermostat in a wireless network inaccordance with a duty cycle, the duty cycle having a connect time inwhich the thermostat is connected in the wireless network and a sleeptime in which the thermostat is not connected in the wireless network.The method includes stealing power through an “on-mode” load of theclimate control system; using the stolen power for charging an energystorage device to provide the power for the wireless network interface;and adjusting at least the sleep time in accordance with a time forcharging the energy storage device to a threshold voltage.

Further areas of applicability will become apparent from the descriptionprovided herein. The description and specific examples in this summaryare intended for purposes of illustration only and are not intended tolimit the scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustrative purposes only ofselected embodiments and not all possible implementations, and are notintended to limit the scope of the present disclosure.

FIG. 1 is a diagram of an exemplary climate control system including athermostat in accordance with an exemplary embodiment of the presentdisclosure;

FIG. 2 is a diagram of an exemplary power stealing circuit in accordancewith an exemplary embodiment of the present disclosure; and

FIG. 3 is a flow diagram of a control-performed method for controllingwireless thermostat network connections in accordance with an exemplaryimplementation of the present disclosure.

Corresponding reference numerals indicate corresponding parts throughoutthe several views of the drawings.

DETAILED DESCRIPTION

Example embodiments will now be described more fully with reference tothe accompanying drawings.

The inventors hereof have recognized that currently availablewireless-enabled thermostats typically require continuous power. Variousdigital thermostats utilize “off-mode” power stealing to obtainoperating power. That is, when a climate control system load (e.g., acompressor, fan, or gas valve) has been switched off, power may bestolen from the “off-mode” load's circuit to power the thermostat. Awireless-enabled thermostat, however, consumes more power than generallywould be available from batteries or from currently used methods ofpower stealing. Wireless-enabled thermostats typically are connected toa climate control system transformer “hot” wire (e.g., an R wire) and acommon wire (e.g., a C wire), so that the full transformer voltage mightbe accessed to power the thermostat's wireless capability.

The inventors also have observed, however, that most current gas furnaceapplications do not make use of a C wire. Also, it often happens that aC wire is not provided in a wire bundle that has been run from a climatecontrol system transformer to a thermostat. In such cases, a homeowneror contractor may find it necessary to pull a C wire, e.g., throughwalls and/or other infrastructure in order to be able to install awireless-enabled thermostat.

Accordingly, the inventors have developed and disclose herein exemplaryembodiments of apparatus, systems and methods of power stealing for awireless-enabled thermostat. In some embodiments, a wireless-enabledthermostat is provided in which a wireless interface may be powered bypower stealing from a transformer through a load that is in an “on”mode, e.g., through a compressor when the thermostat is calling forcooling, and/or, e.g., through a gas valve when the thermostat iscalling for heat.

With reference now to the figures, FIG. 1 illustrates an exemplaryembodiment of a climate control system 10 embodying one or more aspectsof the present disclosure. As shown in FIG. 1, the climate controlsystem 10 includes two power sources, e.g., two transformers 14 and 18for providing power respectively to a heating subsystem 22 and a coolingsubsystem 24. The heating subsystem transformer 14 has a hot (typically24-volt) side 28 and a common, i.e., neutral, side 30. The coolingsubsystem transformer 18 has a hot (typically 24-volt) side 32 and acommon, i.e., neutral, side 34. The cooling subsystem 24 includes a fan38 and a compressor 42 connected on the common side 34 of thetransformer 18. The heating subsystem 22 includes a furnace gas valve 46connected on the common side 30 of the heating subsystem transformer 14.In the present example, a C terminal is provided from a common C wireconnected, e.g., with the common side 34 of the transformer 18. Invarious embodiments of the disclosure, no C wire is used or evenprovided.

In one example embodiment, a thermostat 50 is provided for controllingoperation of the climate control system 10. The thermostat 50 mayactivate one or more relays and/or other switching devices(s) (not shownin FIG. 1) to activate the heating subsystem 22 or cooling subsystem 24.When, e.g., a user operates the thermostat 50 to cause the climatecontrol system 10 to provide heating, the thermostat 50 turns on theheating subsystem 22 and gas valve 46 by using a relay or otherswitching device to connect a “hot” terminal RH to a load terminal W. Toprovide cooling, the thermostat 50 may turn on the compressor 42 and/orfan 38 by using one or more relays or other switching device(s) toconnect a “hot” terminal RC to load terminals Y and/or G.

An example power stealing circuit 60 may obtain power from thetransformers 14 and/or 18 for the thermostat 50. In various embodimentsof the disclosure, and as further described below, the power stealingcircuit 60 may utilize “on-mode” power stealing. Stolen power may beused for powering one or more components of the thermostat 50, includingbut not necessarily limited to a control 64 having a processor 66 andmemory 68, and a wireless network interface 70, e.g., a networkinterface card (NIC). The wireless network interface 70 is configured toprovide intermittent connection of the thermostat 50 in a wirelessnetwork, e.g., a home network through which the user may communicatewith an energy management service and manage climate control in the homeusing a wireless connection with the thermostat 50. In variousembodiments, the thermostat 50 may be connected with the wireless homenetwork in accordance with a duty cycle. The duty cycle has a connecttime in which the thermostat 50 is connected in the wireless network,and a sleep time in which the thermostat 50 is in a “sleep” state, i.e.,not connected in the wireless network. For example, the thermostat 50may be connected with the wireless network for a connect time, e.g., of30 seconds, after which the thermostat 50 sleeps for a sleep time of,e.g., 30 seconds, and so on.

The power stealing circuit 60 is configured to steal power through an“on-mode” load of the climate control system 10 to provide power for atleast the wireless network interface 70 during connect times of the dutycycle as further described below. It should be noted generally thatthermostat embodiments and/or power stealing circuit embodiments inaccordance with various aspects of the disclosure could be installed inother types of climate control systems, including but not limited tosystems having a single transformer, heat-only systems, cool-onlysystems, heat pump systems, etc. In some embodiments a C terminal may beprovided, e.g., from the common side 30 of the transformer 14. In someother embodiments, a thermostat may not be provided with a connection toa common C wire. Further, the climate control system 10 shown in FIG. 1provides single-stage heat and single-stage cooling. But in otherembodiments, a thermostat having a power stealing circuit as describedherein may be provided in a climate control system having multiplestages of heating and/or cooling.

An example embodiment of a power stealing circuit is indicated in FIG. 2by reference number 100. The power stealing circuit 100 may be adaptedfor use in a thermostat for any one of a plurality of climate controlsystem types, e.g., systems having a single transformer, two-transformersystems, heat-only systems, cool-only systems, heat pump systems, etc.The power stealing circuit 100 may be configured to steal power throughone or more climate control system loads. In various embodiments, thepower stealing circuit 100 is configured to steal power from a load thatis in an “on” mode.

In the example embodiment shown in FIG. 2, the power stealing circuit100 is configured to steal power, e.g., from a heating subsystemcontrolled through a thermostat in which the power stealing circuit 100is included. When, e.g., a user operates the thermostat to provideheating, a thermostat control 104 turns on the heating subsystem gasvalve by closing relay(s) 108 to connect a “hot” terminal RH to a loadterminal W. The power stealing circuit 100 includes a current sensor116, e.g., a clipper circuit, a current transformer 120, e.g., astep-down current transformer, and a rectifier 124, e.g., a full waverectifier. In various embodiments, another current transformer typecould be used, including (without limitation) a step-up transformer. Insome embodiments, a half-wave rectifier could be used. The rectifier 124is connected across a high-voltage capacitor 130 (or other energystorage device) and a regulator circuit 134, e.g., a buck circuit, toprovide an output 138 to a wireless network interface 140 of thethermostat. The regulator circuit 134 may be configured to reduce thecapacitor voltage to a level compatible with the wireless networkinterface 140. The output 138 may be, e.g., a 3.3 VDC voltage.

The capacitor 130 becomes charged as rectified current flows through therectifier 124. When the capacitor 130 is charged, e.g., to a thresholdvoltage, the regulator circuit 134 outputs power sufficient to make andat least temporarily maintain a network connection through the networkinterface 140. In various embodiments, the capacitor 130 typically takesa maximum of 30 seconds to charge to the threshold voltage. A voltageVmon across the capacitor 130 is monitored by the control 104, whichcontrols the wireless network interface 140 based at least in part onthe capacitor voltage. The control 104 also controls the wirelessnetwork interface 140 based at least in part on a duty cycle aspreviously discussed with reference to FIG. 1. In some embodiments, thecontrol 104 may switch the regulator circuit 134 on or off based on themonitored voltage Vmon. In some other embodiments in which the regulatorcircuit 134 operates in an open-loop mode and is allowed by the control104 to remain energized, the control 104 may directly enable or disablethe wireless network interface 140 based on the monitored voltage Vmon.

One example control-performed method for controlling thermostat networkconnections through a network interface is indicated generally in FIG. 3by reference number 200 and shall be described with reference to thepower stealing circuit 100 shown in FIG. 2. In process 204, the control104 sets a timer for a duty cycle sleep time, e.g., for 30 seconds, andcauses the thermostat to “sleep.” In process 208, it is determinedwhether the timer has expired and/or whether the capacitor 130 hasbecome charged to a threshold voltage. In either or both events, thethermostat is caused in process 212 to “wake up,” to set the timer forthe duty cycle connect time of, e.g., 30 seconds, to connect with thewireless network, and possibly to send and/or receive informationthrough the network. Thus, the control 104 may adjust at least the sleeptime period in accordance with a time for charging the capacitor 130 tothe threshold voltage. When the timer expires, control may return toprocess 204 in which the timer is reset and the thermostat returns tosleep. If in process 208 the timer is still active and the capacitor isnot yet charged to the threshold voltage, then control returns toprocess 208.

As load current increases on the W wire, there is more current availablefrom the current transformer 120. The capacitor 130 thus can be chargedmore quickly and can reach the threshold voltage more quickly than whena lower amount of current is available. At such times, the thermostatcan be cycled more quickly through network connection cycles and canspend more time connected in the network than through adherence to onlythe predetermined duty cycle. In this way, performance of the examplemethod 200 automatically adjusts the cycle times for networkconnections. The more power that can be stolen, the more frequently thenetwork connections can be cycled, thereby reducing latency time toand/or from the network. In various embodiments, various capacitors andthreshold capacitor voltages may be used in relation to, e.g., varioustypes of regulator circuits, including but not limited to buck circuits,boost circuits, converter circuits, integrated circuits, etc.Additionally or alternatively, various duty cycles could be provided,e.g., to accommodate various capacitor charging times. In variousembodiments, a thermostat may include a battery to provide backup powerin the event that power stealing is not available. Additionally oralternatively, power can be stolen, e.g., from both “on-mode” and“off-mode” loads at the same time in some example embodiments.

Embodiments of the foregoing thermostats and power stealing circuits donot require a common wire to provide power for a wireless capability.Instead, a capacitor of a thermostat power stealing circuit can be usedas a power reservoir that charges and discharges so that intermittentwireless network connections for network updates can be provided to thethermostat. In climate control systems in which both heating and coolingloads are available for power stealing, a wireless thermostat radiocould be powered almost continuously (i.e., powered during duty cycleconnection time periods that are long compared to sleep time periods.)In addition, exemplary embodiments are disclosed that include acapacitor. In alternative embodiments, other energy storage devices maybe used besides capacitors, such as rechargeable batteries, etc.

Example embodiments are provided so that this disclosure will bethorough, and will fully convey the scope to those who are skilled inthe art. Numerous specific details are set forth such as examples ofspecific components, devices, and methods, to provide a thoroughunderstanding of embodiments of the present disclosure. It will beapparent to those skilled in the art that specific details need not beemployed, that example embodiments may be embodied in many differentforms, and that neither should be construed to limit the scope of thedisclosure. In some example embodiments, well-known processes,well-known device structures, and well-known technologies are notdescribed in detail. In addition, advantages and improvements that maybe achieved with one or more exemplary embodiments of the presentdisclosure are provided for purpose of illustration only and do notlimit the scope of the present disclosure, as exemplary embodimentsdisclosed herein may provide all or none of the above mentionedadvantages and improvements and still fall within the scope of thepresent disclosure.

Specific dimensions, specific materials, and/or specific shapesdisclosed herein are example in nature and do not limit the scope of thepresent disclosure. The disclosure herein of particular values andparticular ranges of values for given parameters are not exclusive ofother values and ranges of values that may be useful in one or more ofthe examples disclosed herein. Moreover, it is envisioned that any twoparticular values for a specific parameter stated herein may define theendpoints of a range of values that may be suitable for the givenparameter (i.e., the disclosure of a first value and a second value fora given parameter can be interpreted as disclosing that any valuebetween the first and second values could also be employed for the givenparameter). For example, if Parameter X is exemplified herein to havevalue A and also exemplified to have value Z, it is envisioned thatparameter X may have a range of values from about A to about Z.Similarly, it is envisioned that disclosure of two or more ranges ofvalues for a parameter (whether such ranges are nested, overlapping ordistinct) subsume all possible combination of ranges for the value thatmight be claimed using endpoints of the disclosed ranges. For example,if parameter X is exemplified herein to have values in the range of1-10, or 2-9, or 3-8, it is also envisioned that Parameter X may haveother ranges of values including 1-9, 1-8, 1-3, 1-2, 2-10, 2-8, 2-3,3-10, and 3-9.

The terminology used herein is for the purpose of describing particularexample embodiments only and is not intended to be limiting. As usedherein, the singular forms “a,” “an,” and “the” may be intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. The terms “comprises,” “comprising,” “including,” and“having,” are inclusive and therefore specify the presence of statedfeatures, integers, steps, operations, elements, and/or components, butdo not preclude the presence or addition of one or more other features,integers, steps, operations, elements, components, and/or groupsthereof. The method steps, processes, and operations described hereinare not to be construed as necessarily requiring their performance inthe particular order discussed or illustrated, unless specificallyidentified as an order of performance. It is also to be understood thatadditional or alternative steps may be employed.

When an element or layer is referred to as being “on,” “engaged to,”“connected to,” or “coupled to” another element or layer, it may bedirectly on, engaged, connected or coupled to the other element orlayer, or intervening elements or layers may be present. In contrast,when an element is referred to as being “directly on,” “directly engagedto,” “directly connected to,” or “directly coupled to” another elementor layer, there may be no intervening elements or layers present. Otherwords used to describe the relationship between elements should beinterpreted in a like fashion (e.g., “between” versus “directlybetween,” “adjacent” versus “directly adjacent,” etc.). As used herein,the term “and/or” includes any and all combinations of one or more ofthe associated listed items.

The term “about” when applied to values indicates that the calculationor the measurement allows some slight imprecision in the value (withsome approach to exactness in the value; approximately or reasonablyclose to the value; nearly). If, for some reason, the imprecisionprovided by “about” is not otherwise understood in the art with thisordinary meaning, then “about” as used herein indicates at leastvariations that may arise from ordinary methods of measuring or usingsuch parameters. For example, the terms “generally,” “about,” and“substantially,” may be used herein to mean within manufacturingtolerances.

Although the terms first, second, third, etc. may be used herein todescribe various elements, components, regions, layers and/or sections,these elements, components, regions, layers and/or sections should notbe limited by these terms. These terms may be only used to distinguishone element, component, region, layer or section from another region,layer or section. Terms such as “first,” “second,” and other numericalterms when used herein do not imply a sequence or order unless clearlyindicated by the context. Thus, a first element, component, region,layer or section discussed below could be termed a second element,component, region, layer or section without departing from the teachingsof the example embodiments.

Spatially relative terms, such as “inner,” “outer,” “beneath,” “below,”“lower,” “above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. Spatiallyrelative terms may be intended to encompass different orientations ofthe device in use or operation in addition to the orientation depictedin the figures. For example, if the device in the figures is turnedover, elements described as “below” or “beneath” other elements orfeatures would then be oriented “above” the other elements or features.Thus, the example term “below” can encompass both an orientation ofabove and below. The device may be otherwise oriented (rotated 90degrees or at other orientations) and the spatially relative descriptorsused herein interpreted accordingly.

The foregoing description of the embodiments has been provided forpurposes of illustration and description. It is not intended to beexhaustive or to limit the disclosure. Individual elements, intended orstated uses, or features of a particular embodiment are generally notlimited to that particular embodiment, but, where applicable, areinterchangeable and can be used in a selected embodiment, even if notspecifically shown or described. The same may also be varied in manyways. Such variations are not to be regarded as a departure from thedisclosure, and all such modifications are intended to be includedwithin the scope of the disclosure.

What is claimed is:
 1. A wireless-enabled thermostat for use in aclimate control system, the thermostat comprising: a control having awireless network interface configured to intermittently connect thethermostat in a wireless network in accordance with a duty cycle, theduty cycle having a connect time in which the thermostat is connected inthe wireless network and a sleep time in which the thermostat is notconnected in the wireless network; and a power stealing circuitconfigured to steal power through an “on-mode” load of the climatecontrol system and to charge a capacitor to provide the power for thewireless network interface, the power stealing circuit having a currenttransformer in series with a relay operable to switch the load between“on” and “off” modes, the current transformer configured to obtaincurrent from the “on-mode” load for charging the capacitor; the controlconfigured to adjust at least the sleep time in accordance with a timefor charging the capacitor to a threshold voltage.
 2. The thermostat ofclaim 1, wherein the time for charging the capacitor varies with currentthrough the “on-mode” load.
 3. The thermostat of claim 1, wherein thecontrol is configured to monitor the charge of the capacitor and controlthe wireless network interface based on the monitoring.
 4. Thethermostat of claim 1, wherein the power stealing circuit comprises: aclipper circuit connected across the current transformer; a rectifierconnected between the current transformer and the capacitor andconfigured to charge the capacitor from the current transformer; and aregulator circuit for powering the wireless network interface, theregulator circuit powered by the capacitor.
 5. The thermostat of claim4, wherein the regulator circuit comprises a buck circuit, and/or thecontrol is configured to monitor the charge of the capacitor and controlthe regulator circuit based on the monitoring.
 6. The thermostat ofclaim 1, wherein the power stealing circuit is configured to steal powerwithout using a climate control system common “C” wire.
 7. Thethermostat of claim 1, wherein the control is configured to provideintermittent connection of the thermostat with the wireless networksubstantially only when power from the power stealing circuit isavailable.
 8. The thermostat of claim 1, wherein the control isconfigured to disconnect the thermostat from the wireless network whenpower from the power stealing circuit has become unavailable, andreconnect the thermostat in the wireless network when power from thepower stealing circuit has been restored.
 9. The thermostat of claim 1,wherein the current transformer comprises a step-up current transformer.10. A wireless-enabled thermostat for use in a climate control system,the thermostat comprising: a control having a wireless network interfaceconfigured to intermittently connect the thermostat in a wirelessnetwork in accordance with a duty cycle, the duty cycle having a connecttime in which the thermostat is connected in the wireless network and asleep time in which the thermostat is not connected in the wirelessnetwork; and a power stealing circuit configured to steal power throughat least an “on-mode” load of the climate control system and to chargean energy storage device over a time period varying with current throughthe “on-mode” load, to provide the power for the wireless networkinterface, the power stealing circuit having a current transformer inseries with a relay operable to switch the load between “on” and “off”modes, the current transformer configured to obtain current from the“on-mode” load for charging the energy storage device; the controlconfigured to adjust at least the sleep time in accordance with thevarying time period.
 11. The thermostat of claim 10, wherein the controlis configured to monitor the charge of the energy storage device andenable or disable the wireless network interface based on themonitoring.
 12. The thermostat of claim 10, wherein the power stealingcircuit comprises: a clipper circuit connected across the currenttransformer; a rectifier connected between the current transformer andthe energy storage device and configured to charge the energy storagedevice from the current transformer; and a regulator circuit forpowering the wireless network interface, the regulator circuit poweredby the energy storage device.
 13. The thermostat of claim 12, whereinthe regulator circuit comprises a buck circuit and/or the currenttransformer comprises a step-up current transformer.
 14. The thermostatof claim 12, wherein the regulator circuit is configured to operate inone or more of the following modes: open loop, and under control of thecontrol.
 15. The thermostat of claim 10, wherein the control isconfigured to provide intermittent connection of the thermostat with thewireless network substantially only when power from the power stealingcircuit is available.
 16. The thermostat of claim 10, wherein thecontrol is configured to disconnect the thermostat from the wirelessnetwork when power from the power stealing circuit has becomeunavailable, and reconnect the thermostat in the wireless network whenpower from the power stealing circuit has been restored.
 17. Thethermostat of claim 10, wherein the sleep time decreases with increasingcurrent through the “on-mode” load.
 18. A method of communicating in awireless network, the method performed by a wireless thermostat of aclimate control system, the method comprising: controlling a wirelessnetwork interface of the thermostat to intermittently connect thethermostat in a wireless network in accordance with a duty cycle, theduty cycle having a connect time in which the thermostat is connected inthe wireless network and a sleep time in which the thermostat is notconnected in the wireless network; stealing power through at least an“on-mode” load of the climate control system, the stealing performed atleast in part by receiving current from the “on-mode” load from acurrent transformer in series with a relay operable to switch the loadbetween “on” and “off” modes and using the received current for chargingan energy storage device to provide the power for the wireless networkinterface, where the current being received from the current transformerfor the charging increases as current increases through the “on-mode”load; and adjusting at least the sleep time in accordance with a timefor charging the energy storage device to a threshold voltage.
 19. Themethod of claim 18, wherein the sleep time period decreases withincreasing current through the “on-mode” load.
 20. The method of claim18, wherein: the energy storage device comprises a capacitor; and themethod further comprises: determining whether a duty cycle timer hasexpired and/or whether the capacitor is charged to the thresholdvoltage; and based on the determining, initiating a connect time andresetting the timer.